- . Jury 9, 1914] 
NATURE 
497 
Now comes a very interesting point. When Moseley 
sets the increasing atomic weights against the corre- 
spondingly decreasing wave-lengths, the changes do 
not run exactly parallel with each other. But if the 
wave-lengths are compared with a series of natural 
numbers everything runs smoothly. In fact, it is 
obvious that the steady decrease in the wave-length 
as we pass from atom to atom of the series in the 
periodic table implies that some fundamental element 
of atomic structure is altering by equal steps. There 
is excellent reason to believe that the change consists 
in successive additions of the unit electric charge to 
the nucleus of the atom. We are led to think of the 
magnitude of the nucleus of any element as being 
simply proportional to the number indicating the place 
of the element in the periodic table, hydrogen having 
a nuclear charge of one unit, helium two, and so on. 
The atomic weights of the successive elements do not 
increase in an orderly way; they mount by steps of 
about two, but not very regularly, and sometimes they 
seem absolutely to get into the wrong order. For 
example, nickel has an atomic weight of 58-7, whereas 
certain chemical properties, and, still more, its be- 
haviour in experiments on radio-activity indicate that 
it should lie between cobalt (59) and copper (63-6). 
But the wave-lengths, which are now our means of 
comparison, diminish with absolute steadiness in the 
order cobalt, nickel, copper. Plainly, the atomic num- 
ber is a more fundamental index of quality. than the 
atomic weight. 
It is very interesting to find, in the series arranged 
in this way, three, and only three, gaps which remain 
to be filled by elements yet undiscovered. 
Let us now glance at another and most important 
side of the recent work, the determination of crystal- 
line structure. We have already referred to the case 
of the rock-salt series, but we may look at it a little 
more closely in order to show the procedure of crystal 
analysis. 
The reflection of a pencil of homogeneous rays by a 
set of crystalline planes occurs, as already said, at a 
series of angles regularly increasing, giving, as we 
say, spectra of the first, second, third orders, and so 
on. When the planes are all exactly alike, and equally 
spaced, the intensities of the spectra decrease rapidly 
as we proceed to higher orders, according to a law 
not yet fully explained. This is, for example, the case 
with the three most important sets of planes of 
sylvine, those perpendicular to the cube edge, the face 
diagonal and the cube diagonal respectively. An 
examination of the arrangement of the atoms in the 
simple cubical array of sylvine shows that for all these 
sets the planes are evenly spaced and similar to each 
other. It is to be remembered that the potassium 
atom and the chlorine atom are so nearly equal in 
weight that they may be -onsidered effectively equal. 
In the case of rock-salt the same may be said of the 
first two sets of planes, but not of the third. The 
planes perpendicular to the cube diagonal are all 
equally spaced, but they are not all of equal effect. 
They contain alternately, chlorine atoms (atomic 
weight 35-5) only, and sodium atoms (atomic weight 
23) only. The effect of this irregularity on the intensi- 
ties of the spectra of different orders is to enhance 
the second, fourth, and so on in comparison with the 
first, third, and fifth. The analogous effect in the 
case of light is given by a grating in which the-lines 
are alternately light and heavy. A grating specially 
ruled for us at the National Physical Laboratory shows 
this effect very well. This difference between rock- 
‘salt and sylvine and its explanation in this way con- 
stituted an important link in W. L. Bragg’s argument 
as to their structure. 
When, therefore, we are observing the reflections 
NO. 2332, VOL. 93] 
in the different faces of a crystal in order to obtain 
data for the determination of its structure, we have 
more than the values of the angles of reflection to help 
us; we have also variations of the relative intensities 
of the spectra. In the case just described we have an 
example of the effect produced by want of similarity 
between the planes, which are, however, uniformly 
spaced. ; 
In the diamond, on the other hand, we have an 
example of an effect due to a peculiar arrangement of 
planes which are otherwise similar. The diamond 
crystallises in the form of a tetrahedron. When any 
of the four faces of such a figure are used to reflect 
X-rays, it is found that the second order spectrum 
is missing. The analogous optical effect can be 
obtained by ruling a grating so that, as compared 
with a regular grating of the usual kind, the first and 
second, fifth and sixth, ninth and tenth, alone are 
drawn. To put it another way, two are drawn, two 
left out, two drawn, two left out, and so on. The 
National Physical Laboratory has ruled a special grat- 
ing of this kind also for us, and the effect is obvious. 
The corresponding inference in the case of the diamond 
is that the planes parallel:to any tetrahedral face are 
spaced in the same way as the lines of the grating. 
Every plane is three times as far from its neighbour 
on one side as from its neighbour on the other.. There 
is only one way to arrange the carbon atoms of the 
crystal so that this may be true. Every atom is at 
the centre of a regular tetrahedron composed of its 
four nearest neighbours, an arrangement best realised 
by the aid of a model. It is a beautifully simple and 
uniform arrangement, and it is no matter of surprise 
that the symmetry of the diamond is of so high an 
order. Perhaps we may see also in the perfect sym- 
metry and consequent effectiveness of the forces which 
bind each atom to its place an explanation of the hard- 
ness of the crystal. 
Here, then, we have an example of the way in 
which peculiarities of spacing can be detected. There 
are other crystals in which want of uniformity, both 
in the spacings and in the effective values of the 
planes, combine to give cases still more complicated. Of 
these are iron pyrites, calcite, quartz, and many others. 
It would take too long to explain in detail the method 
by which the structures of a large number of crystals 
have already been determined. Yet the work done 
so far is only a fragment of the whole, and it will 
take no doubt many years, even though our methods 
improve as we go on, before the structures of the most 
complicated crystals are satisfactorily determined. 
On this side then we see the beginning of a new 
crystallography which, though it draws freely on the 
knowledge of the old, yet builds on a firmer foundation 
since it concerns itself with the actual arrangement 
of the atoms rather than the outward form of the 
crystal itself. We can compare with the internal 
arrangements we have now discovered the external 
forms which crystals assume in growth, and the modes 
in which they tend to come apart under the action of 
solvents and other agents By showing how atoms 
arrange and disarrange themselves under innumerable 
variations of circumstances we must gain knowledge 
of the nature and play of the forces that bind the 
atoms together. 
There is yet a third direction in which inquiry may 
be made, though as yet we are only at the beginning 
of it. In the section just considered we have thought 
of the atoms as at rest. But they are actually in 
motion, and the position of an atom to which we have 
referred so frequently must be an average position 
about which it is in constant movement. Since the 
atoms are never exactly in their places, the precision of 
the joint action on which the reflection effect depends 
